Many known alarm systems incorporate wired circuits for purposes of communicating with and/or providing electrical energy to various types of detectors and/or output devices such as sounders, strobe lights and the like all without limitation. The nature of such systems is such that a supervisory function is often provided for such wiring. Such are often monitored to determine if they are exhibiting either gross open circuit or short circuit conditions.
One prior art solution has been to terminate the spur wiring with an end of line resistor. The value of the resistor is such that it will not affect operation of output appliances such as sounders or strobe lights or the like when the spur has been energized in response to a detected alarm condition. In the absence of an alarm condition, the polarity on the spur can be reversed by the local control unit and a monitoring current provided which can be expected to flow only through the end of line resistor under normal operating circumstances.
A gross open circuit can be readily detected since the monitoring current cannot flow through the output appliances in a reverse polarity condition. Similarly, a gross short circuit can be detected as the supervisory current will exceed a normal expected value.
FIG. 1 illustrates a prior art alarm system 10 which incorporates a system control unit 12. Unit 12 is in communication with a plurality of detectors 14 via a wired medium 16. As is conventional, the detectors 14 can communicate their status relative to one or more sensed ambient conditions to the control unit 12 for evaluation.
Control unit 12 can also incorporate a switchable power supply 20 which is coupled to a wired output medium 22. A plurality of output devices 26 can be coupled across the output medium 22. The output medium 22 terminates in an end of line resister 30.
In normal operation, a non-alarm state, the system 10 via the switchable power supply 20 can couple a supervisory current Isup through end of line resister 30 with a polarity as indicated relative to resister 30. With the indicated polarity, the output devices or appliances 26 which could be strobe lights, horns, sirens, bells, sounders all without limitation are inactive and non responsive. Both the presence of a gross short across the wired medium 26 or an open circuit therein can be detected by the value of the supervisory current.
In the event of an alarm condition the power supply 20 reverses polarity and energizes the output appliances 26 via a drive current Id as indicated. In this condition the loop, medium 26, is not being supervised.
However, end of line resistors, and supervisory currents, as described above, while useful may not detect all such faults. For example, a high series impedance, not an open circuit, in the spur or a low parallel impedance, not a short circuit, across the spur may not be detectable using known resistor end of line techniques. In such circumstances, while the system may appear to be fault free, it may not be able to activate all of the output devices when required.
Thus there is a continuing need for supervisory circuits and methods which can detect partial fault conditions of the type described above. Preferably such circuitry and methods could be implemented without substantial additional costs and would also be preferably retrofittable on or to previously installed systems.